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Characterization of Acoustic Transients: Calibration and Standardization Robert Burkard University at Buffalo 18 November 2011 Why quantify/verify/calibrate acoustic signals? -To relate percept or physiologic response to acoustic stimuli- as we cannot directly measure ‘hearing’, which is subjective. -So we can compare results across clinics/laboratories. What if all audiometers were calibrated to differently. How would we compare an audiogram from San Diego to that from Buffalo? Let us start with a definition of Sound Pressure Level (SPL) From ANSI S3.20-1995 American National Standard Bioacoustical Terminology: “Ten times the logarithm to the base ten of the ratio of the time-mean-square of a sound, in a stated frequency band, to the square of the reference sound pressure in gases of 20 µPa. Unit, decibel (dB); abbreviation: SPL;” dB SPL = 10log(P2/20µ Pa2) or, more familiarly: dB SPL = 20log(P/20µ Pa) Where this comes from (Speaks, 1996): A Bel is the log of the ratio of two sound powers (in Watts), or sound intensities (in Watts/m2, or Watts/cm2): Bel = logI1/I2 and a tenth of a Bel (or deciBel, dB) is: dB = 10log(I1/I2) Some data many years ago suggested that the lowest human threshold was near 10-12 W/m2, so: dB IL = 10log(I/10-12 W/m2) Again from Speaks (1996): I = Prms2/ρos Where I is sound intensity, P is pressure, and ρo is ambient density of the medium, and s is the speed of sound in the medium; ρos is known as characteristic impedance of the medium. As this will factor out later, let us drop characteristic impedance: I ~ Prms2 Also, at room temperature and at sea level, 10-12 W/m2 is equal to 20 µPa; substituting: dB IL = 10log(I/10-12 W/m2) dB SPL = 10log(P2/20 µPa2) dB SPL = 10log(P/20 µPa)2 dB SPL = 20log(P/20 µPa) Another definition from ANSI S3.20-1995: Peak frequency-weighted sound pressure level: ‘Greatest instantaneous value of standard frequencyweighted sound pressure level, within a stated time interval. Unit, decibel (dB),’ So peak SPL is: pSPL = 20log(Pp/20 µPa), where Pp is peak Pressure Relevant to this presentation, the crest factor of a signal is the ratio of its peak pressure to its rms pressure. For a sine wave, the crest factor is 1.414, or 20log1.414 = 3 dB Most measure SPL with a sound level meter. When using either a TDH-39, -49 or -50, or an, e.g., Etymotic ER3A insert earphone, this measurement system is comprised of: A coupler (6 cc or 2 cc), a type I measurement microphone (typically condenser), an input amplifier, filtering circuit, output amplifier, and some form of display circuit. Although the 6 cc and 2 cc couplers cannot be considered artificial ears, they are commercially available, and allow similar measurements to be made in different locations. from Curtis and Schultz, 1986 from B&K 1971, 1982 from Johnson, Marsh and Harris, 1998 Fast: 125 ms τ Slow: 1000 ms τ Impulse: 35 ms τ for signals increasing in level, 1500 ms τ for signals decreasing in level Peak: instantaneous largest pressure; hold B&K 1986 Filtering from B&K Frequency Analysis, 1977 from Johnson, Marsh, and Harris, 1998 For TDH-39, -49, -50 earphonesUse a 6-cc coupler (several types); For Etymotic (ER3A) earphones-use a 2-cc coupler (also several types) from B&K Data Handbook, 1982 from Haughton, 2002 Instead of SLM, can use a microphone, preamplifier, conditioning amplifier from B&K Data Handbook, 1982 from Johnson, Marsh, and Harris, 1998 Condenser microphone -Most require 200 V polarization voltage -may require voltage amplifier, to increase voltage -the larger the microphone (1”, ½”, ¼”, 1/8”), the greater the sensitivity -The smaller the microphone, the higher the upper cutoff frequency Sensitivity: mV/Pa: 46.8 mV/Pa 20log.0468 = -26.6 Microphone- flat to 2 kHz, up to 1 dB more sensitive from 2-7 kHz, less sensitive above 7 kHz Now that we know about SLMs and other sound level measurement equipment, how do we accurately measure the sound pressure level of a transient? Some possibilities: -Increase the rate of the transients, using the fast time weighting -Measure peak SPL, using the peak-hold mode, if the SLM has this capability -Measure peak SPL using a microphone, preamplifier, and conditioning amplifier -Use the peak-equivalent SPL procedure -Increase the rate of the transients, using the fast time weighting If you increase click rate in the ‘fast’ meter mode, there is an increase in SPL with increasing rate from Burkard 1984 Peak SPL (pSPL) If you have a SLM with a ‘peak hold’ capability, Route the earphone through the SLM, set the ‘peak hold’, and reset to obtain a new measure. -Problem 1: Response is to the largest instantaneous sound pressurewhether signal or noise. -Problem 2: The time constant should be as short as possible (several tens of µs or less) for an accurate estimate of pSPL of clicks. -Problem 3: ‘Impulse’ is not the same as ‘peak’Impulse: 35 ms exponential τ for sounds increasing in level over time, and an exponential τ of 1500 ms for sounds decreasing in level over time. Peak SPL: with microphone, preamp, conditioning amplifier and oscilloscope Simply route the transducer through the coupler, microphone, preamp, conditioning amplifier to oscilloscope. Two calibration/measurement methods: 1. Microphone sensitivity 2. Acoustic calibrator Microphone sensitivity: 1. Measure peak voltage of transient 2. Using microphone sensitivity, convert measured voltage to pressure 3. Use the dB SPL formula to convert to pSPL Sensitivity: 50 mV/Pa Measured voltage: 10 mV (peak) 50 mV/1 Pa = 10 mV/ X Pa; 10 mV/50 mV = X Pa/1 Pa X = .2 Pa dB pSPL = 20log (.2 Pa/.00002 Pa) = 80 dB pSPL Be careful, many conditioning amplifiers have adjustable gain. Same microphone: Microphone sensitivity 50 mV/Pa 40 dB gain in conditioning amplifier Measure 1 volt peak voltage for click: 40 dB = 20logX, X = 100x gain 1000 mV/100 = 10 mV 50 mV/Pa = 10 mV/X Pa; X = .2 Pa dB pSPL = 20log(.02 Pa/.00002 Pa) = 80 dB pSPL Or 50 mV/Pa = 1000 mV/X Pa; X = 20 Pa dB pSPL = 20log(20 Pa/.0002 Pa) – 40 dB pSPL= 120 – 40 = 80 dB pSPL Using Acoustic Calibrator (or Pistonphone) 1. Place calibrator on microphone 2. Record rms voltage 3. Relate pressure (or SPL) to voltage Use same microphone: 50 mV/Pa sensitivity Place 114 dB SPL calibrator, and measure 500 mV 114 = 20 log(P/.00002 Pa) 5.7 = log(P/.00002 Pa) 501187 = P/.00002 Pa P = 10.02 Pa 500 mV/10 Pa = 50 mV/Pa Peak-Equivalent Sound Pressure Level (peSPL): i. Route output of the SLM to an oscilloscope ii. Measure amplitude of the, e.g., click (Vp, Vp-p) iii. Route sine wave into earphone, adjust level of sine wave until Vp or Vp-p is equal to that measured in /ii/ iv. Read SPL on readout of SLM If Vp-p is 2X Vp (undamped), then dB peSPL and dB p-peSPL are equal If Vp-p is equal to Vp (critically damped), then dB pSPL is 6 dB greater than dB p-peSPL. from Burkard and Secor, 2002 Relationship between several methods for measuring level of transients From Burkard 1984 A Comparison of Different Measures of Sound Pressure Level (SPL) for Click Stimuli in Both Supra-aural and Insert Earphones Robert Burkard 27 June 2011 (IERASG) Introduction IEC 60645-3 recommends using peak-to-peak peak equivalent sound pressure level (p-p peSPL) for the calibration of acoustic transients. RETSPLs of transients recommended by ISO 389-6 are based on p-p peSPL. A review of the literature reveals that calibration of transients does include the use of p-p peSPL, but also uses baseline-to-peak peSPL (b-p peSPL) as well as peak SPL (pSPL). The relationship between b-p peSPL and pSPL is fixed (that is, the crest factor of a sine wave: 20log1.414 = 3.02 dB). However, the numerical relationship between p-p peSPL and b-p pe SPL can vary over a range of 6.04 dB, making it impossible to know the relationship between pSPL and p-p peSPL without empirical measurements dB p-p peSPL can range from 0 to 6 dB less than dB b-p peSPL As Chair of ANSI S3/WG71 “Auditory Evoked Potentials”, I have been struggling with how to include the various measures of transient level (p-p peSPL, b-p peSPL and pSPL) in the ANSI Standard, and yet still harmonize with the international standard. I made some acoustic calibration of both supra-aural and insert earphones, using a variety of couplers and microphones, to ascertain the relationship between p-p peSPL and b-p peSPL. These values, for various earphones and measurement systems may be considered for inclusion as Annex material in the forthcoming ANSI standard Grass S88 to TDT current amplifier to earphone (and Tektronix o-scope) Earphone to coupler to microphone to LD824 SLM to o-scope Frequency responses: SRS Spectrum analyzer to current driver to earphone to coupler/microphone/SLM Peak Equivalent SPL Measure rms voltage (Vref), AC output of SLM to 1 Pa acoustic calibrator (94 dB SPL) Measure bpV: dB pSPL = 20log(bpV/Vref) + 94 dB b-p peSPL = 20log (bpV/Vref/1.414) + 94 Measure ppV: dB p-p peSPL = 20log(ppV/Vref/2.828) + 94 A bpV ppV bpV ppV B C IEC 318 #2 (BB); 2 cc couplers: HA 2 1” and ½”, Occluded ear simulator NBS 9A IEC 318 TM HA-2, 2 cc coupler, 1” microphone 2 cc, HA-2, ½” microphone HA-1, 2 cc, ½” microphone Occluded Ear Simulator, with rigid tube Occluded Ear Simulator Foam Insert Earphones: 6 ER-3A from Etymotic (thank you): 2 10 ohm, 2 50 ohm, 2 300 ohm (soft tube) 7 EAR insert earphones: various impedances, both soft and more rigid tubing Supra-aural Earphones: 6 TDH-39s, 3 TDH-49s, 3 TDH-50s (various impedances) Measurements: For pSPL/peSPL: Adjust voltage level until pSPL on SLM is ~100 dB Measure b-pV and p-pV. Redo 3 times, for each coupler and earphone Calculate pSPL, b-p peSPL, p-p peSPL from voltage measurements, and calculate mean values Frequency response: Supra-aural: NBS-9A, LD 1” mic: Constant voltage to all earphones. For inserts: HA-2 (BB) TM 1/2” mic: Adjust voltage so that near 1000 Hz, ~105 dB SPL. Vary frequency at this voltage, in 1/3 octave steps from 100-~10,000 Hz. Constant voltage (NBS-9A, 1” mic uncorrected) TDH-39 TDH-49 115 110 110 39-1 39-3 100 39-4 95 dB SPL dB SPL 105 39-2 105 49-1 100 49-2 49-3 39-5 95 39-6 90 85 90 100 1000 10000 100 Frequency (Hz) TDH-50 10000 Earphone Comparison 110 110 105 105 50-1 100 50-2 50-3 95 dB SPL dB SPL 1000 Frequency (Hz) TDH-39 100 TDH-49 TDH-50 95 90 90 100 1000 Frequency (Hz) 10000 100 1000 Frequency (Hz) 10000 Etymotic versus EAR: constant SPL @ 1008 Hz Insert 11 - 17 Insert 1 - 6 120 120 110 Insert-4 80 Insert-5 Insert-6 70 Insert-13 90 Insert-14 Insert-15 80 Insert-16 70 Insert-17 60 60 100 1000 10000 100 100000 1000 10000 Frequency (Hz) Frequency (Hz) Insert Comparison 120 110 100 dB SPL dB SPL Insert-3 90 Insert-12 100 dB SPL Insert-2 100 Insert-11 110 Insert-1 Insert 1 - 6 90 Insert 11 - 17 80 70 60 100 1000 10000 Frequency (Hz) 100000 100000 Peak SPL (oscilloscope) – Peak SPL (SLM) Supra-aural Earphones NBS-9A (BB) mean min max TDH-39 -0.63 -1.77 -0.10 TDH-49 0 -0.3 +0.3 TDH-50 -0.4 -0.03 -0.6 NBS-9A(TM) mean min max -0.51 -1.35 +0.1 +0.06 -0.05 +0.13 -0.43 -0.18 -0.58 IEC318 (BB) mean min max -0.37 -1.63 +0.2 +0.14 -0.13 +0.37 -0.34 -0.30 -0.47 IEC318 (TM) mean min max +0.56 +0.30 +0.80 +0.38 +0.17 +0.57 +0.40 +0.30 +0.47 Baseline-to-Peak peSPL – Peak-to-Peak peSPL Supra-aural Earphones NBS-9A (BB) mean min max TDH-39 3.15 2.7 3.87 TDH-49 3.94 3.30 4.47 TDH-50 5.24 5.17 5.33 NBS-9A(TM) mean min max 2.40 1.80 2.68 2.90 2.48 3.30 3.87 3.75 4.00 IEC318 (BB) mean min max 3.50 2.77 3.83 3.38 3.03 3.67 4.00 3.53 4.30 IEC318 (TM) mean min max 3.39 2.77 3.80 2.82 2.33 3.27 3.78 3.53 4.03 Peak SPL (oscilloscope) – Peak SPL (SLM) Insert Earphones HA-2 1” BB mean min max Etymotic +0.33 +0.27 +0.40 EAR +0.29 +0.20 +0.40 Etymotic HA-1 1/2”(BB) mean +0.03 min -0.03 max +0.07 EAR +0.08 +0.03 +0.13 HA-2 1” TM mean min max +0.28 +0.23 +0.33 +0.28 +0.23 +0.30 Occluded Ear mean +0.09 Sim- BB-rigid min +0.03 tube max +0.17 +0.12 +0.10 +0.17 HA-2 ½” BB mean min max +0.22 +0.13 +0.37 +0.19 +0.13 +0.27 Occluded Ear mean +0.17 Sim- BB-foam min +0.10 max +0.23 +0.19 +0.10 +0.30 HA-2 ½” TM mean min max -0.09 -0.20 0 -0.40 -0.73 -0.16 Baseline-to-Peak peSPL – Peak-to-Peak peSPL Insert Earphones HA-2 1” BB mean min max Etymotic 2.69 2.60 2.80 EAR 2.59 2.23 3.00 Etymotic HA-1 1/2”(BB) mean 4.66 min 4.27 max 4.90 EAR 4.45 3.63 5.07 HA-2 1” TM mean min max 2.61 2.37 2.77 2.59 2.23 2.97 Occluded Ear mean 1.23 Sim- BB-rigid min 1.17 tube max 1.37 1.15 .90 1.43 HA-2 ½” BB mean min max 2.45 2.37 2.57 2.35 1.93 2.73 Occluded Ear mean 1.49 Sim- BB-foam min 1.20 max 1.67 1.59 1.17 2.00 HA-2 ½” TM mean min max 2.32 2.10 2.60 2.20 1.70 2.63 Summary: pSPL/peSPL measures 1. pSPL as measured by the LD 824 SLM using the peak hold mode (Flat) typically produces a click level within several tenths of a dB from that observed using the AC output of the SLM, using procedures similar to those used to measure peSPL. 2. For the supra-aural earphone measures, there was a substantially greater range of differences between the two pSPL measures, as compared to that observed for the insert earphones. 3. Sources of differences between the two measures may include: a. The integration time required for the SLM peak-detector circuit, which would lead to a lower SPL in the peak-hold value of the SLM. b. Spurious noise in the room or from the earphone. This could lead to a high-level transient that could produce a value from the SLM peakhold circuit that exceeds that using the peSPL approach. 4. The difference in peSPL using the p-p and b-p approaches varies across earphone type, coupler used, and microphone used. There is also some variation across individual transducers within a specific type of transducer. 5. Thus, to convert from b-p peSPL to p-p peSPL, or vice versa, one must have conversion values for all transducers used for AEP clinical efforts, and for all couplers (and possibly earphones) used to make pe SPL measurements. As couplers do not provide an exact SPL measurement at the tympanic membrane: How about ‘real-ear’ measures? Speaks 1996 The problem with ‘real-ear’ measures is that at frequencies above a few kHz, there are standing waves in the ear canal, and the sound pressure measurement at the probe microphone may not accurately reflect the SPL at the plane of the TM Other Calibration Issues for Transients: Spectral Analyses: In most cases, we will route the output of the microphone/amplifier or SLM to a digital spectrum analyzer. We will often low-pass filter the input to spectrum analyzer (an antialiasing filter; may be included in the spectrum analyzer) For continuous signals (like tones), we will typically pass this through a windowing function, like a Hanning or Flattop window. This ‘window’ attenuates the response at the onset and offset, to prevent spectral leakage For transients, should use a Uniform or Rectangular window (equal weighting), and capture entire transient in the time window of analyzer, to prevent unintentional changing of the spectrum of the transient Why are ‘clicks’ typically 100 µs pulses for humans? from Durrant 1983 from Burkard 1984 Linear/Log spectrum: envelope effects from Burkard 1984 Toneburst duration bandwidth (Hz)! 500 Hz 1000 Hz 2000 Hz 4000 Hz 5 ms 5 ms 5 ms 5 ms 410-610 Hz: 200 Hz 905-1105 Hz: 200 Hz 1903-2103 Hz: 200 Hz 3901-4101 Hz: 200 Hz .57 .288 .144 .072 500 Hz 1000 Hz 2000 Hz 4000 Hz 4 ms 2 ms 1 ms 0.5 ms 390-640 Hz: 250 Hz 780-1280 Hz: 500 Hz 1562-2562 Hz:1000 Hz 3123-5123 Hz: 2000 Hz .71 .71 .71 .71 !: CF = (Fl x Fu)0.5 #octaves* * #octaves: (log{Fu/Fl})/log2 Should you use a constant time envelope or a constant number of cycles envelope across toneburst frequency? 1. The cochlea is scaled logarithmically (~5 mm/octave) in at least the mid- to high-frequencies. If you wanted to stimulate a constant proportion of the cochlear partition, a constant number of cycles (producing a constant number of octaves) is preferable. 2. Using constant-slope stimuli, Suzuki and Horiuchi (1982) found that the ABR is elicited by the first 1-2 cycles of the envelope- again supporting the notion of using a constant number of cycles (perhaps a 2-1-2 cycle rise-plateau-fall time) Some Final Thoughts: -Reports often are vague about calibration methods used of acoustic transients. pSPL can range from 3 to 9 dB greater than peSPL. -In all cases, you should state the instrumentation and technique used to determine the SPL of transients. -In my opinion, it is best to use the baseline-to-peak peSPL method, as it can easily be converted to pSPL (by adding 3 dB). To perform a similar conversion using peak-to-peak peSPL requires knowledge of the impulse symmetry. However, the current international standard recommends the peak-to-peak peSPL approach (IEC 645) -We should state, in all reports, the normative population used to determine a 0 dB nHL value for transient stimuli (i.e., the RETSPL). This must include the rate at which the stimuli are presented (temporal integration affects perceptual threshold but not AEP threshold), and subject characteristics (including audiometric criteria). Some quantification of the ambient noise conditions of of test room is useful. -Although ISO 389-6 has provided RETSPLs for clicks and ‘standard’ tonebursts, there is no equivalent ANSI document for RETSPLs of transients. Thus, it is imperative that we quantify our signals as precisely as possible. It is likely that 35 dB nHL values can vary by more than 5 dB across labs (or clinics), and this 5 dB difference will likely have a substantial influence on pass/fail rates in infant screening programs. These calibration issues are even more problematic in AEP studies in rodents (such as mice) -The earphones used for humans are limited in bandwidth, as mouse hearing often extends well above 40 kHz -A 2-cc coupler does not represent a reasonable approximation to the volume (or the input impedance) of a mouse ear -A 1” microphone starts rolling off below 10 kHz, and smaller microphones (e.g., ¼” or 1/8”) are preferable -Often a SLM is designed for the human audiofrequency range (up to 20 kHz), and attenuates signals above this range We cannot directly measure sensation, and to determine hearing ability, we must pair the signal with the response. Those who use AEPs to evaluate the hearing of, e.g., newborns, know the challenges of recording a repeatable AEP in a clinical environment. It is equally important to pay attention to the acoustic stimulus used in these evaluations, as this will affect of results of your hearing evaluation. Questions?